Building efficiency factors and net-to-gross ratio

One-line orientation

Overall Building Efficiency shows what share of the gross building area is net assignable program space. Divide the required net assignable area by this ratio to estimate the gross building area.

Key points

• Overall Building Efficiency = NAA / GBA. The primary programming tool: gives the fraction of gross floor area that is actually usable for programmed spaces. Influenced by the building’s use type (e.g., hospitals and labs are less efficient than offices because they require more mechanical/service space).
• Interior Layout Efficiency = NAA / Usable Area. Measures how efficiently the interior fit-out converts rented or occupied floor area into assignable space. Useful for estimating interior construction costs.
• Base Building Efficiency = Usable Area / GBA. Reflects the core-and-shell design: how much of the gross footprint the building structure and core makes available to occupants. Used to determine GBA when the Usable requirement is known.
• Sizing a building from the net program: Gross = Net ÷ Overall Building Efficiency. To “gross up” a net program, divide Net Assignable Area (NAA) by the overall efficiency. Dividing by a number below 1 makes the required Gross Building Area (GBA) larger.
• Pro Practice framing: Programming is typically the owner’s responsibility, often with architect or consultant assistance. The deliverables usually include a net-to-gross ratio, cost-per-SF estimate, site requirements, and mechanical/HVAC load projections.
• Cost-per-SF: Applied to GBA (gross area), not NAA, because contractors price by the full building footprint. The programming team uses the net-to-gross ratio to arrive at GBA first, then applies cost-per-SF for the budget estimate.

Building efficiency: three ratios, one area hierarchy

First identify the numerator and denominator; then choose the relationship.

View diagram Hide diagram

Scroll horizontally to explore →

To size the building: GBA = NAA ÷ Overall Efficiency — divide, never multiply.

Schematic containment only — the insets do not encode typical efficiency values.

Confusions / comparison

Efficiency factor	Formula	What it estimates	Used to calculate
Overall Building Efficiency	NAA / GBA	What fraction of the total building is assignable space	GBA from NAA: GBA = NAA ÷ factor
Interior Layout Efficiency	NAA / Usable Area	How well the interior fit-out converts occupied area to program space	Interior construction cost benchmarks
Base Building Efficiency	Usable Area / GBA	How much of the gross building core-and-shell design delivers to occupants	GBA from Usable: GBA = Usable ÷ factor

Related

→ prog-area-types (this module): definitions of GBA, Usable, Rentable, and NAA — required reading before this card · prog-criteria-to-block-diagram (this module): how program areas become sized block diagrams · Code — allowable area and FAR (Code module): zoning caps on GBA.